Universal Mobile Telecommunications System (UMTS) is an example of a mobile radio communications system. UMTS is a 3rd Generation (3G) mobile communication system employing Wideband Code Division Multiple Access (WCDMA) technology standardized within the 3rd Generation Partnership Project (3GPP). In the 3GPP release 99, the radio network controller (RNC) in the radio access network controls radio resources and user mobility. Resource control includes admission control, congestion control, and channel switching which corresponds to changing the data rate of a connection. Base stations, called node Bs (NBs), which are connected to an RNC, orchestrate radio communications with mobile radio stations over an air interface. The RNC controls what system information the Node B should broadcast and is the control plane protocol termination point towards user equipments (UEs). RNCs are also connected to nodes in a core network, i.e., Serving GPRS Support Node (SGSN), Gateway GPRS Support Node (GGSN), mobile switching center (MSC), etc. Core network nodes provide various services to mobile radio users who are connected by the radio access network such as authentication, call routing, charging, service invocation, and access to other networks like the Internet, public switched telephone network (PSTN), Integrated Services Digital Network (ISDN), etc.
The Long Term Evolution (LTE) of UMTS is under development by the 3rd Generation Partnership Project (3GPP) which standardizes UMTS. There are many technical specifications hosted at the 3GPP website relating to Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN), e.g., 3GPP TS 36.300. The objective of the LTE standardization work is to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology. In particular, LTE aims to support services provided from the packet switched (PS)-domain. A key goal of the 3GPP LTE technology is to enable high-speed packet communications at or above about 100 Mbps.
FIG. 1 illustrates an example of an LTE type mobile communications system 10. An E-UTRAN 12 includes E-UTRAN NodeBs (eNBs) 18 that provide E-UTRA user plane and control plane protocol terminations towards the user equipment (UE) terminals 20 over a radio interface. The eNB controls radio resources and user mobility. An eNB is sometimes more generally referred to as a base station, and a UE is sometimes referred to as a mobile radio terminal or a mobile station. As shown in FIG. 1, the base stations are interconnected with each other by an X2 interface. The base stations are also connected by an S1 interface to an Evolved Packet Core (EPC) 14 which includes a Mobility Management Entity (MME) and to a System Architecture Evolution (SAE) Gateway. The MME/SAE Gateway is shown as a single node 22 in this example and is analogous in many ways to an SGSN/GGSN gateway in UMTS and in GSM/EDGE. The S1 interface supports a many-to-many relation between MMEs/SAE Gateways and eNBs. The E-UTRAN 12 and EPC 14 together form a Public Land Mobile Network (PLMN). The MMEs/SAE Gateways 22 are connected to directly or indirectly to the Internet 16 and to other networks. In the following description, the control plane protocol termination point towards the UEs is called the RAN node. So depending on the RAN configuration, an eNB, RNC, collapsed RNC, and Node B may all be such a RAN node.
There are times when a core network node, like the MME or the SGSN, may be overloaded with control plane (CP) messages because too many mobile users are sending too many messages in a particular time frame. Assuming the core network node queues incoming messages, then the load on the node could be measured based on queue length. If the time that the core network node needs to process the incoming messages is longer than the arrival rate of new messages in the time frame, then the queue length increases. When the queue length passes some limit or when the queue is full, the core network node may consider itself to be overloaded. In that state, the core network node is not able to process the incoming messages in a reasonable time, and there is a risk that some messages may be lost due to queue overflow. Typically, there is one queue for all messages to the core network node, and hence, each radio access network node connected to a core network node should be informed about the overload status. As a result, an overload message must be sent to the radio access network requesting that the signaling load be reduced.
The problem is how best to do this in a way that effectively reduces the CP signaling load on one or more core network nodes but also implements this reduction so that network coverage and capacity arc not adversely affected. For example, it would be undesirable to have one radio access network node dramatically cut back on new and/or existing UE connections in an effort to reduce the CP signaling load at the core network node while other radio access network nodes experience little or no cut back as a response to identical overload messages. Nor would it be beneficial to employ a system where mobile radios leave one cell because of actions to reduce CP signaling load, but then simply transfer that same load via a nearby cell resulting in no reduction of the load in the core network node.
Moreover, a core network node may want to avoid sending an overload message to all radio access nodes once the core network node is overloaded because sending each such message requires processing resources, which at that point are overloaded with processing requests. It would be preferable for the core network node to start sending the messages out while it “still has resources”, i.e., while processing power can be used for processing out going messages and not all processing power is required to process the queued incoming messages. Depending on load status and how much and how fast the load increases, the core network node may need to adapt the amount of CP load reduction.
Another problem is that when a RAN node is generally instructed to reduce its CP load, that RAN node does not know how much it needs to reduce that load or the reason why. For example, if there is a disaster like an earthquake, then the RAN nodes might want to restrict “normal” communications from all or at least most mobile terminals to make sure that emergency personnel have access and emergency calls can get through. Current guidelines for standardized core networks overload mechanisms simply indicate that a “step size” reduction of the signaling load should be performed if an overload condition exists. But there is no guidance on how that should be interpreted or implemented.